(1) Field of the Invention
This invention relates to heating devices and, more particularly, to heating devices molded of conductive loaded resin-based materials comprising micron conductive powders, micron conductive fibers, or a combination thereof, homogenized within a base resin when molded. This manufacturing process yields a conductive part or material usable within the EMF or electronic spectrum(s).
(2) Description of the Prior Art
From common kitchen appliances to sophisticated temperature control devices for scientific application, resistive heating elements are ubiquitous in application. Most heating elements are highly resistive metal wire, such as nickel-chromium (nichrome) or tungsten, designed to provide the necessary resistance for the heating required. The resistance of the heating element is determined by the resistivity of the wire, its cross-sectional area, and its length. The heat generated by the heating element is determined by the current passing through the heating element. Typically, the heating element further comprises an outer layer of a material that serves as an electrical insulator and a thermal conductor.
Heat generated in a resistive heating element is transferred to heated objects by conduction, convection and/or radiation. Conduction heat transfer relies on direct contact between the heating element and the heated object. For example, the transfer of heat from an electric range to a metal pan is essentially by conduction. Convection heat transfer relies on fluid flow to transfer heat. For example, an egg cooking a pan of boiling water relies on convection currents to transfer heat from the metal pan through the water and to the egg. Water at the bottom of the pan is superheated causing it to lose density such that it rises. This rising superheated water transfers heat energy to the egg floating in the water. Conversely, the water at the top of the pan is cooler and denser and, therefore, falls to toward the bottom of the pan. A convection current is thereby established in the pan of water. Radiation heat transfer relies on electromagnetic energy (such as light) to transfer heat from the heating element to the object. For example, a cake baking in an electric oven will be heated, in part, by the radiated heat from the glowing heating element. Radiant heating in how the sun's energy reaches the earth. In practical application, the three means of thermal transfer are found to interact and frequently occur at the same time.
Resistive heating elements used in various heating systems and applications have advantages over, for example, combustion-based heating sources. Electric heating elements do not generate noxious or asphyxiating fumes. Electric heating elements may be precisely controlled by electrical signals and, further, by digital circuits. Electrical heating elements can be formed into many shapes. Very focused heating can be created with minimal heat exposure for nearby objects. Heating can be performed in the absence of oxygen. Fluids, even combustible fluids, can be heated by properly designed resistive heating elements.
However, resistive heating elements currently used in the art have disadvantages. Metal-based elements, and particularly nichrome and tungsten, can be brittle and therefore not suitable for applications requiring a flexible heating element. Further, the large thermal cycles inherent in many product applications and the brittleness of these materials will cause thermal fatigue. Other metal elements, such as copper-based elements, bring greater flexibility. However, if the application requires the resistive element to change or flex positions, then the resistive element will tend to wear out due to metal fatigue. Metal-based resistive heating elements are typically formed as metal wires. These elements are expensive, can require very high temperature processing, and are limited in shape. In addition, when a breakage occurs, typically due to fatigue as described above, then the entire element stops working and must be replaced.
Several prior art inventions relate to electrically conductive plastics. U.S. Pat. No. 4,197,218 to McKaveney describes electrically conductive articles. The articles are formed from a non-conductive matrix containing an electrically conductive dispersion of finely divided ferroalloy, silicon alloy, or mixtures. U.S. Pat. No. 5,771,027 to Marks et al describes a composite antenna with a grid comprised of electrical conductors woven into the warp of a resin reinforced cloth forming one layer of the multi-layer laminate structure of the antenna. U.S. Pat. No. 6,249,261 to Solberg, Jr. et al details a direction-finding antenna constructed from polymer composite materials that are electrically conductive. The polymer composite materials replace traditional metal materials. U.S. Pat. No. 6,277,303 to Foulger describes conductive polymer composite materials. The conductive polymer composite material includes a minor phase material that has a semi crystalline polymer. The composite material further includes a conductive filler material dispersed in the minor phase material in an amount sufficient to be equal to or greater than an amount required to generate a continuous conductive network within the minor phase material. The composite material also incorporates a major phase material. The major phase material being a polymer which when mixed with the minor phase material will not engage in electrostatic interactions that promote miscibility. The major phase material has the minor phase material dispersed within it in an amount sufficient to be equal to or greater than an amount required to generate a continuous conductive network in the major phase material. This composite then forms a semiconductive ternary composite with distinct co-continuous phases.
U.S. Pat. No. 6,558,746 to Starz et al details a coating composition for producing electrically conductive coatings, containing one or more electrically conductive pigment and an organic binder. The coating composition, optionally, contains additives and auxiliary agents. The coatings thus obtained are especially well-bonded and resistant to mechanical influences and to solvents, and exhibit suitable conductivity (sheet resistivity) values. U.S. Pat. No. 6,602,446 to Ushijima provides an electrically conductive paste made up of an electrically conductive filler combined with a heating element adapted to generate heat upon electromagnetic induction. The paste is then compounded with a resin. In addition, Nv Bekaert sa of Kortrijk, Belgium manufactures metal yarns, knitted metal fabric, chopped metal fibers and pellets, and sintered porous media. The fibers are marketed with diameters of from 1 μm to 20 μm and may be chopped into fiber pieces or be of continuous yarns. The metals shown in the product description found www.bekaert.com Jan. 25, 2003 are stainless steel, temperature resistant alloys, nickel and nickel alloys, titanium, aluminum, and copper. In the article, “I Want My Pizza Hot!,” by Sbenaty, in the Journal of Science, Technology, Engineering and Math Education, January-April 2000, Volume 1, Issue 1 a design exercise describes a heating element for home delivery of food such as Pizza using a material that consists of a flexible conductive polymer material connected between integral copper bus wires.
In the article, “Nanocomposite Materials Offer Higher Conductivity and Flexibility”, McCluskey, et al., Proceedings of 3rd International Conference on Adhesive Joining and Coating Technology in Electronics Manufacturing, 1998, pp: 282-286, describes the mechanical and electrical characteristics of a conductive polymer made with conductive silver flake nanoparticle fillers. The use of nanoparticle fillers allows the material to attain the same level of conductivity exhibited by traditional filled polymers at significantly lower particle loading. The conductive polymer combines the high conductivity and stability of a filled polymer with the flexibility and low density of an intrinsically conductive polymer. The nanoparticle metal fillers examined have dimensions between 200 nm and 20 μm, and when mixed with a non-conductive polymer matrix, they have a resistivity of from 10-100 Ohm-cm. Further, McCluskey et al. discusses that the onset of conductivity of a silver filled silicone begins with 65-75% ratio by weight of silver to the silicone for a 3 μm-20 μm particle size. A 200 nm particle has an onset of conductivity at 35-40% ratio by weight of silver to the silicone.